DESCRIPTION (From applicant's abstract): Listeners with sensorineural hearing impairment frequently show deficits in speech understanding, particularly in difficult listening situations involving competing sounds. There may be especially large deficits relative to normal-hearing listeners in the context of dynamically-varied maskers such as speech from a single additional talker or reverberation - the types of maskers commonly found in the real world. These perceptual deficits do not reflect simply the absolute sensitivity loss and reduced audibility of the target speech. The research proposed here focuses on influences of cochlear damage on auditory processing of signals that are audible to listeners with sensorineural hearing loss. In normal-hearing listeners, outer hair cells within the cochlea actively influence cochlear mechanics through a process referred to as the cochlear amplifier or active mechanism. Reduced influence of the active mechanism on internal processing may contribute significantly to the deficits that hearing-impaired listeners experience in identifying complex sounds like speech in competing sound environments. The proposed studies are directed studying the influence of the active mechanism on the processing of audible signals and a means of assessing active mechanism status in individual hearing-impaired listeners. The first set of experiments evaluate a possible psychoacoustic means of assessing active mechanism status in normal-hearing and hearing-impaired listeners. Later studies apply these findings to tasks involving speech-like material and to running speech in order to clarify the role of the active mechanism in the processing of speech in competing sound.